Hydro-pneumatic mechanisms are known in which the fluid side of a hydro-pneumatic accumulator is connected to the cylinder of an hydraulic ram to impose a restraint on the movement of the piston of the ram when the piston is subjected to external influences. Such prior art assemblies have been used, for example, in the suspensions of motor vehicles as shock absorbers, and have been used for similar purposes in other apparatus and equipment.
In such prior art hydro-pneumatic mechanisms, the fluid connection between the accumulator and the ram is permanently open, so that the ram is at all times subjected to the influence of the accumulator. The compensating apparatus of the present invention, however, is predicated upon modifying the operation of the ram by controlling the fluid connection between the accumulator and the ram. With such a control, compensating apparatus can be provided which is of particular utility in certain environments, and which can, if so desired, be constructed to derive energy from the environment itself to perform certain operations.
For example, apparatus constructed in accordance with the concepts of the present invention, may be used as a wave-motion compensator in conjunction, for example, with a crane on an offshore oil drilling rig for loading or unloading a vessel which is rising and falling with respect to the rig assembly. More specifically, the apparatus of the present invention in the embodiment to be described, may be coupled to the load-carrying cable of the crane in such a manner that the crane cable is initially maintained at a substantially constant slight tension as the load is rising and falling relative to the crane, and then serves to pick the load off the crest of a wave to permit it to be hoisted onto the rig.
In the embodiment of the wave-motion compensating apparatus to be described, the fluid coupling between the accumulator and the ram is maintained open for a particular interval to permit the load to rise and fall relative to the crane, and the coupling is then closed so that the load is no longer able to fall with the wave motion, but is taken up and supported by the crane. The combination of the invention, as will be described, is also capable of then introducing energy into the ram, so as to enable the compensating apparatus to continue to move the load upwardly from the crest of the wave. Energy derived from the wave motion itself is used to charge the separate source in the embodiment to be described, so that the apparatus may be self-contained and require no additional external energy source.